First, you need to switch to Slide Master in order to see the layouts available in your powerpoint presentation.
Now, hover the mouse over the layout where you want to know how many Slides are using it. You will see that a tooltip appear saying that the given layout is used by slide(s) and the number of Slides that are using the layout.
If no layouts are using it, then you will read that. This tip can be really helpful if you need to know what layouts are used and what other aren’t.